The appearance of printed text or graphics produced by thermal inkjet print heads varies if the viscosity of the ink changes. Viscosity is affected by the printhead temperature which in turn varies with the use profile of the printhead and the temperature environment in which the printer operates.
One prior art approach taken in dealing with this problem has been to provide a spittoon into which ink drops are ejected prior to commencing printing. The purpose of this is twofold. First such ink drop ejection tends to clear viscous plugs from the nozzle of the thermal inkjet printhead and second, this preliminary use of the printhead provides a warm up interval, hopefully to achieve a printhead temperature at or near a desired temperature for printing purposes.
Another prior art effort in dealing with this problem has been to provide a multi-grade ink in which the change is viscosity over a limited range of printhead operating temperatures would not result in significant degradation of print quality.